The video camera is provided with a zoom lens located at the position of the coated surface of the film. The zoom lens is formed of a fixed cylinder, an outer movable cylinder, and an inner movable cylinder, which house a first group of lenses and a second group of lenses. The focus positions of these lenses change along with the telescopic displacements of the movable cylinders. When the camera is in use, these lenses are mounted on a wide-angle position or a telephoto position.
The zoom lens must be mounted in the body of the video camera in such a manner that the focal position of the lens must be consistent with the coated surface of the film. This calls for series of operations with regard to the optical position, so as to make sure that the focal position of the lens is located on the coated surface of the film loaded in the body of the camera. In order to detect the precise optical position, it is necessary to measure mechanically the data of a retraction position of the lens under the circumstance that the main switch of the camera is turned off. When the lens is located at the wide-angle position, the data of a wide-angle position of the lens are mechanically measured. Finally, the data of a telephoto position of the lens are mechanically measured at the time when the lens of the camera is set at the telephoto position. The data are then keyed into an EEPROM on the camera body loop so as to enable the lens to stop at a specified position.
Such a method of data measurement as described above is deficient in design in that the reference data measured by means of instrument can not be efficiently employed in practice, and that the measured data are often erroneous in view of the inaccurate reference position of locating the lens at the time of measurement. In addition, an additional space must be reserved at each position of the lens, thereby resulting in an excessive increase in volume of the camera.